The prior art contains a number of patents directed to flow rate determining devices for liquid flowing in a liquid system. For example, U.S. Pat. No. 2,101,257, issued Dec. 7, 1937 to M. Vogel-Jorgensen, for "Apparatus for Measuring Liquid or Fluent Materials" discloses a measuring device for cement slurries fed to a rotary kiln. The concept is to provide a calibrated volume which may be placed into or out of the normal flow line for the cement slurry, such that a flow rate may be taken at intervals in order to monitor the rate at which the cement slurry is fed. This arrangement does not, of course, give an accurate totalized volume of flow over an entire flow period, but merely allows the instantaneous rate to be monitored from time to time. From these instantaneous rates, of course, an approximate idea may be had of the total volume passing through the flow line, but exact precision is not possible.
A similar problem arises with U.S. Pat. No. 2,392,951, R. G. Salisbury, issued Jan. 14, 1946 and entitled "Flow Meter". Here again, a calibrated volume is adapted to be placed into or out of connection with a line along which a fluid is continually flowing, in order to be able to monitor the rate of flow at specific intervals. However, a totalized and accurate flow volume over a continuous time period cannot be obtained by the use of the flow meter disclosed in this patent.
My own earlier U.S. Pat. No. 4,455,870, issued June 26, 1984 and entitled "Method and Apparatus for Determining Liquid Flow Rates" represents a particular solution to the problems inherent in the prior art. My previous patent provides a method of determining the total inflow of a liquid through a liquid-flow system in which the liquid enters a sump cavity and is pumped out of the sump cavity by pump means. A computing means calculates an on-going total inflow volume for the liquid by (i) adding in the sump cavity volume between lower and upper limit levels each time the liquid surface rises to the upper limit level, and (ii) determining the inflow rate over the last portion of the filling time just described and extrapolating this inflow rate over the time when the pump means is pumping to yield an incremental quantity, such quantity being added in to the on-going total inflow volume.
The method set forth in my earlier patent, while providing a satisfactory procedure for determining the total inflow volume, is nonetheless not absolutely accurate particularly in cases where, during the pumping cycle, the inflow rate changes more or less dramatically. In other words, the extrapolation of a previously measured rate through the pumping cycle, without taking into account a variation of the inflow rate during the pumping, can lead to an error.
In view of this problem, it is an aspect of the present invention to allow a more accurate determination of the true inflow rate during the pumping cycle, thus attaining greater accuracy in the calculation of the total inflow of the liquid.